The invention relates generally to computer systems, and more specifically to a program tool to optimize program code for execution.
Computer programs often include conditional evaluations based on the value of a variable, such as “If X=A, then go to step 10000; otherwise go to step 10010. The condition can be true or false depending on the current value of the variable “X”. For example, a variable “debug” is set to “true” when a program is in a debugging stage. There is a computer program which conditionally evaluates the variable “debug” to determine whether certain data, useful for debugging purposes, should be logged. In other words, the state of the debug variable is used in a decision step to determine whether to log the data. The decision step can be: “If debug=true, then write XYZ data to log; otherwise jump to next step”. Thus, if the debug variable is set to “true”, then the specified data is written to the log. However, if the program is not in the debug stage, for example, the program has already complete debugging, then there is no need to write the specified data to the log. So, after the debugging stage, the program developer uses a known program tool to set the variable “debug” to “false” so the XYZ data will not be written to the log. Also, the program developer will use this tool to declare that the “debug” variable is now fixed (or immutable) as “false”. In Sun Microsystems Java (™) programming language and other runtime languages, this declaration will be entered by the programmer as a command written into the source code. The command means that once the variable is set for the first time to the desired fixed state during execution, it will never be changed. During compilation, the compiler records this declaration, monitors this variable, and returns an error code if the value of this variable is attempted to be changed from its initial value.
U.S. Pat. No. 6,728,952 discloses a computer system for identifying predicates that always are true or always are false (i.e. “vacuous”), and states that if the expression on which an IF statement depends is identified as a vacuous predicate, the code for the IF statement may be optimized. This is because the expression on which the IF statement depends will always be true or will always be false. This patent also states that specialized computer systems already exist which are able to determine whether a predicate is vacuous.
A publication entitled “Fast, Effective Dynamic Compilation”, by Auslander et al., SIGPLAN Notices, vol. 31, no. 5, pages 149-159, May 1996, discloses various run time optimization techniques. For example, run-time constants can become instruction immediates rather than memory loads, constant propagation and folding can be applied to them, conditional branches based on them can be eliminated, and loops they control can be fully unrolled.
A publication entitled “Value Profiling” by Calder et al., Proceedings of 13th Annual IEEE/ACM International Symposium on Micro architecture, pages 259-269. 1997 discloses that identification of variables as invariant or constant at compile-time allows the compiler to perform optimizations including constant folding, code specialization and partial evaluation.
An object of the present invention is to further optimize execution of a program.